1. Field of the Invention
The invention relates to a synchrotron radiation source having a beam guidance system for accelerating and storing an electron or positron particle beam on a closed trajectory.
Such synchrotron radiation sources in which, inter alia, magnets formed of superconducting winding configurations are employed, are intended not only for a wide variety of applications in physics research, but they are also used as X-ray sources for purposes of lithography, preferably in the production of semiconductor chips.
Synchrotron radiation is created when an electron or positron particle beam is deflected from a linear trajectory. As a rule, the particle beam is guided (stored) in a beam guidance system on a closed trajectory, and use is made of the synchrotron radiation which is created in the deflecting magnets that are required to bend the trajectory. In order to achieve particularly efficient generation of synchrotron radiation, the trajectory should be bent with the smallest possible radius of curvature. That requires relatively large magnetic fields, which can only be generated in an economic manner for practical purposes, by using superconducting magnets.
2. Description of the Related Art
Synchrotron radiation sources with superconducting magnets are described, for example, in Published European Patent No. 0 208 163 B1, Published European Application No. 0 277 521 A2 and German Published, Non-Prosecuted Application DE 31 48 100 A1. In the simplest case, seen in German Published, Non-Prosecuted Application DE 31 48 100 A1, the synchrotron radiation source includes an electron storage ring with a superconducting magnet system. Such a synchrotron radiation source is especially compact, but the actual realization is difficult, due to the very restricted spatial conditions. Accordingly, it is proposed in Published European Application No. 0 208 163 B1 not to construct the beam guidance system for the electron beam in an annular configuration, but to provide two mutually spaced superconducting deflecting magnets, whereby the particle trajectory is given a "racetrack" form with two linear trajectory sections in which devices for accelerating as well as for injecting and/or extracting the particles may be disposed. Further developments of such a synchrotron radiation source may be inferred, for example, from Published European Application No. 0 277 521 A2.
German Published, Non-Prosecuted Application DE 31 48 100 A1 and Published European Application No. 0 277 521 A2 also contain indications as to the construction of a synchrotron radiation source for application in processes such as X-ray lithography and X-ray microscopy, preferably from the point of view of the selection of the energy of the particles to be stored and the corresponding structure of the magnets. In specific terms, the application of synchrotron radiation sources for the production of integrated circuits or the like with structures in the submicron range, is an important field of industrial application.
Further information on the construction of a synchrotron radiation source, preferably with regard to the structure of the deflecting magnets for the purpose of constructing a non-linear beam optical system, can be inferred from the article entitled "Nonlinear Beam Optics with Real Fields in Compact Storage Rings" by H. O. Moser, B. Krevet and A. J. Dragt, in Nuclear Instruments & Methods in Physics Research/Section B, B30 (1988) Feb. No. 1 pgs. 105-109.
A feature which is a disadvantage of the known configurations in certain circumstances is the problematic handling of the superconducting magnets. On one hand, the most stringent requirements are to be imposed on the mechanical construction of the magnets, which gives rise to correspondingly high production costs, and on the other hand, the application of time-variant current (which is required in the acceleration of a particle beam to a prescribed energy, for example) to superconducting magnets is very difficult, inter alia on account of the eddy currents created in the structures retaining the magnets in this case. Over and above such points, in a beam guidance system for storing a particle beam, as a rule it is desirable to provide devices for focusing the particle beam, in order to ensure good beam properties over relatively long periods of time and to avoid losses of intensity as far as possible. Published United Kingdom Application GB 2 015 821 A discloses a beam guidance system which is constructed with four achromatic deflecting magnets and contains no focusing devices whatsoever. Achromatic deflecting magnets, which can also be referred to as mirror magnets, are described, for example, in the article "Achromatic Magnetic Mirror for Ion Beams" by H. A. Enge, in The Review of Scientific Instruments Vol. 34, No. 4 (1963) pgs. 385-389. A beam guidance system according to Published United Kingdom Application GB 2 015 821 A is not suitable for storing a particle beam over relatively long periods of time, since after a few circuits in the beam guidance system, the particle beam is lost, if it is not previously extracted for onward guidance.
It is accordingly an object of the invention to provide a synchrotron radiation source, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, having a beam guidance system which permits both the acceleration as well as the longer-term storage of an electron or positron particle beam and in which the application of superconducting magnets can be restricted to a very substantial extent.